Cooling system for an automotive engine

ABSTRACT

A cooling system has a radiator for cooling coolant of an automotive engine, cooling fan for cooling the coolant in the radiator, and a thermostat for controlling coolant temperature. The thermostat has a housing having a flange, an actuating rod secured to the housing at a first end thereof, a guide member slidably mounted on the actuating rod, a resilient seal spool provided around a second end portion of the actuating rod and secured to the guide member, a heat sensitive cylinder housing the seal spool and secured to the guide member, wax pellets provided in the heat sensitive cylinder to enclose the seal spool. The flange has a hole so as to pass a coolant, and the thickness of the resilient seal spool being set between 25% and 5% of the diameter of the actuating rod, thereby reducing a spring constant of a return spring of the thermostat. A cooling fan switch is provided for starting the cooling fan at a low coolant temperature.

BACKGROUND OF THE INVENTION

The present invention relates to a cooling system for controllingtemperature of a coolant of an automotive engine.

Referring to FIG. 6 showing a conventional cooling system for anautomotive engine, the system has a thermostat 1 which is disposed in aninlet side passage of water jackets 20.

The cooling system comprises a first coolant passage 24 disposed betweenan upper outlet 21 of the water jackets 20 and an upper inlet 23 of aradiator 22, and a second coolant passage 30 provided between a loweroutlet 25 of the radiator 22 and a lower inlet 29 of the water jackets20, including a thermostat cap 26, a thermostat housing 27 and a waterpump 28. A bypass passage 31 is provided between a junction J of thefirst passage 24 and the thermostat housing 27 so as to communicate thefirst passage 24 with the second passage 30 without passing the radiator22. The thermostat 1 is hermetically secured to the housing 27 by thethermostat cap 26. The thermostat 1 has a main valve 12 for closing thesecond passage 30 and a bypass valve 15 for closing a bypass port 32 ofthe bypass passage 31.

In FIG. 6, the reference A' designates a measuring point for measuringthe temperature of the coolant in the housing 27, and B' designates ameasuring point provided in the second passage 30 adjacent to thethermostat cap 26 and upstream of the thermostat 1 for measuring thetemperature of the coolant in the second passage 30. The reference Cdesignates a measuring point for measuring the flow rate of the coolantin the second passage 30. The reference numeral 33 designates a coolingfan.

The thermostat 1 is operated by a thermo-actuator. The thermo-actuatorcomprises an actuating steel rod and a resilient seal spool which isslidably engaged with the rod. The seal spool is inserted in a heatsensitive cylinder filled with wax pellets.

As shown in FIG. 7, a perforation 19a is formed in a flange 16 of thethermostat 1, and a jiggle valve mechanism 17 having a jiggle valve 18is movably engaged in the perforation 19a.

During the operation of the engine, the jiggle valve 18 is closed by thepressure of the coolant in the second coolant passage 30 as shown inFIG. 7. When the engine stops, the jiggle valve opens. Thus, the coolantcan be supplemented in the direction of the arrow.

During the cold engine state, the main valve 12 of the thermostat 1 isclosed as shown in FIG. 6, and the jiggle valve 18 is also closed by thecoolant pressure, while the bypass valve 15 integrated with the mainvalve 12 is fully opened. Thus, the coolant drawn from the outlet 21 ofthe water jackets 20 does not pass through the radiator 22. The coolantis circulated by the water pump 28 through the junction J of the firstpassage 24, bypass passage 31, housing 27, and inlet 29 of the waterjackets 20 as indicated by arrows. Thus, the temperature of the coolantin the housing 27 quickly rises.

However, since the coolant in the radiator 22 and the thermostat cap 26is not circulated, the temperature rising rate of the coolanttemperature B therein is slow. Therefore, as shown in a record of FIG.8, after even if the temperature A at the point A' becomes 87° C. whichis an opening temperature of the main valve 12, the temperature B at thepoint B' is merely 45° C. There is a difference of 42° C. between thetemperatures A and B.

When the main valve 12 of the thermostat 1 opens, the coolant of a lowtemperature is drawn from the lower outlet 25 of the radiator 22 and fedto the thermostat housing 27 through the second passage 30.Consequently, the temperature B of the coolant at the point B' isfurther lowered by 13° C. As a result, the difference between thetemperature B of the coolant in the passage 30 and the temperature A ofthe coolant in the housing 27 increases to 55° C. The area of the partshown by the hatching indicates energy loss in the period. It will beunderstood that the time of the abscissa indicates the elapse from thetime at 60° C. of the temperature A. Since the heat sensitivity of thethermostat 1 is low, the response of the thermostat delays with respectto the change of the coolant temperature. Therefore, the main valve 12opens after the temperature has become considerably higher than thepredetermined opening temperature 87° C. When the main valve 12 opens,the temperature of the coolant is lowered. The main valve 12 closesafter the coolant temperature has considerably decreased lower than apredetermined closing temperature. Then, the coolant temperature rises.Namely, there is a large heat overshoot in control of the coolanttemperature, so that the main valve is repeatedly opened and closed.When the main valve 12 closes, a surge pressure occurs at the upstreamof the main valve.

The heat overshoot causes cracks of the cylinder block and cylinder headof the engine, and the surge pressure causes breakdown of the thermostat1 and the radiator 22.

Since, mentioned in above, the jiggle valve mechanism is sources ofenergy loss and engine troubles, the jiggle valve mechanism is removedfrom the present thermostat. And a small hole is formed in the flange ofthe thermostat. Therefore, pressures applied to the outer side and innerside of the main valve become equal to each other. The spring constantof the return spring is reduced. As a result, the lift up rate isincreased in a low temperature range. Furthermore, the thickness of theseal spool is extremely thin (thickness of between 25% and 5% of thediameter of the actuating rod), so that the pressure of the wax for thelift up of the valve is reduced.

FIG. 1 is a diagram showing the lift with respect to the coolanttemperature. A line X is the lift of the valve of the present inventionand the line Y is the lift of the conventional valve. The range of thesteep curve line is the solid wax state.

The main valve of the conventional valve Y opens at 72° C., the lift atthe end temperature 87° C. of the solid wax state is merely 9.6 mm.Thereafter, the lift up rate reduces because of the liquid wax state,and when the lift becomes 12 mm, the coolant temperature reaches such ahigh temperature as 123° C.

The main valve of the present invention also opens at 72° C., the liftbecomes 6 mm by a small temperature increase of 9° C. When the liftreaches 12 mm, the coolant temperature is 85° C. as shown by the line X,which is only increase of 4° C. The coolant temperature of 85° C. iswithin the range of the solid wax state.

When the automobile mounting the thermostat of the present invention isdriven at 80 Km/h, the coolant temperature increases to 77.5° C.However, when the speed reaches to 150 Km/h, the coolant temperaturedecreases to 70.5° C., because the radiator is cooled by a strong wind.Therefore, the cooling system using the thermostat of the presentinvention is provided such that the cooling fan switch is closed tostart the cooling fan at an upper limit temperature of 81° C.

The line Z'-Z of FIG. 1 shows the upper limit of 81° C. The hatched areashows the difference between the flow rate of the coolant passing themain valve of the present invention and the flow rate in theconventional valve. The lift of the line X at 81° C. is 6 mm and thelift of the line Y is 3 mm. Therefore, the flow rate of X is two timesas much as the flow rate of Y.

The flow rate at the lift of 6 mm of X which is in the solid wax statecorresponds to the flow rate at the lift of 12 mm of Y which is includedin the liquid wax state. Thus, the thermostat of the present inventionuses only 50% of the own power at the lift of 12 mm. Therefore, even ifan automobile mounting the thermostat of the present invention is drivenat 150 Km/h, there remains the power of 50%.

However, the conventional thermostat of the line Y enters the liquid waxstate range after 86° C. (lift 9.3 mm), where the lift increasing ratelargely decreases. The coolant temperature of 37° C. (123° C.-86° C.) isconsumed in vain in the period from the lift 9.3 to 12 mm. Thethermostat of the present invention demonstrates a double power withhalf of source, remaining power of 50%.

The coolant flowing through a small hole on the flange needs not worryabout taking a longer warm up period for a idling because the cold startfuel injector is provided in the throttle body controlled by a computercompensate a time loss for idling.

In the cooling system, the fan switch of the present invention is linkedto the thermostat, when the coolant temperature reaches 81° C., thecooling fan operates. Since the flow rate of the coolant at 81° C. isthe double of that of the conventional thermostat, the coolanttemperature quickly decreases. Therefore, the coolant temperature iskept 81° C.

The upper limit for the cooling fan is not limited to 81° C. It isdesirable to set the temperature to an effective value as low aspossible dependent on tests.

The high coolant temperature of the conventional thermostat causesvarious problems such as an increase of fuel consumption and aggravationof the emission.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a cooling system havinga wax type thermostat a main valve of which has double flow rate of theconventional thermostat, thereby increasing the thermal efficiency ofthe automotive engine.

In addition, a cooling fan switch is provided to be closed at a coolanttemperature below 81° C. for starting a cooling fan, thereby largelylowering an upper limit temperature of the coolant to an extremely lowtemperature.

According to the present invention, there is provided a cooling systemfor an automotive engine comprising, a radiator for cooling a coolant ofthe engine, a cooling fan for cooling the coolant in the radiator, athermostat having a housing having a flange for attaching thereof to aconduit member, an actuating rod secured to the housing at a first endthereof, a guide member slidably mounted on the actuating rod, aresilient seal spool provided around a second end portion of theactuating rod and hermetically secured to the guide member, a heatsensitive cylinder housing the seal spool and secured to the guidemember, wax pellets provided in the heat sensitive cylinder to enclosethe seal spool, a lubricant oil provided in a space between the sealspool and the actuating rod, a main valve provided on the guide member,and a return spring for urging the main valve to a valve seat formed onthe flange, the improvement comprising, the flange having at least onehole so as to pass a coolant, resulting the spring constant of returnspring is reduced and thickness of the resilient seal spool set between25% and 5% of the diameter of the actuating rod, and a cooling fanswitch for starting the cooling fan at a coolant temperatures below 81°C.

These and other objects and features of the present invention willbecome more apparent from the following detailed description withreference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing changes of a lift of a valve in a thermostatof the present invention and a lift of a conventional valve with respectto temperature;

FIGS. 2 and 3 are sectional side views of the thermostat of the presentinvention;

FIG. 4 is a side view of the thermostat of the present invention;

FIG. 5 shows a sectional view showing a test machine for a valve lift;

FIG. 6 is a schematic diagram of a conventional cooling system for anautomotive engine;

FIG. 7 is a side view of a conventional thermostat;

FIG. 8 is a graph showing changes of temperature and flow rate of thecoolant of the conventional system with respect to time;

FIG. 9a is a sectional view of a cooling fan switch;

FIG. 9b shows a side view showing the cooling fan switch of FIG. 9a atfull-size;

FIG. 10 is a graph showing the change of the coolant temperature withrespect to the elapsed time;

FIG. 11a is a sectional view of an IC cooling fan switch;

FIG. 11b is a side view of the switch at full size; and

FIG. 12 is a schematic diagram showing a cooling system according to thepresent inventions

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 shows a main valve closing state, and FIG. 3 shows a main valveopening state. A thermostat 1a of the present invention has a housing 10forming a valve seat 9 (FIG. 3), a frame 11 secured to a flange 16 ofthe housing 10.

A thermo-actuator 2 comprises an actuating steel rod 3, a guide member 4slidably mounted on the rod 3, and a resilient seal spool 5 which ishermetically secured to the guide member 4 and slidably engaged with therod 3. The thickness of a bag portion of the seal spool 5 is between 25%and 5% of the diameter of the rod 3. A lubricating oil 6 fills the spacebetween the seal spool 5 and the rod 3.

The seal spool 5 is inserted in a heat sensitive cylinder 8 filled withwax pellets 7. An end of the cylinder 8 is securely engaged with theguide member 4, thereby forming the thermo-actuator 2.

The rod 3 of the thermo-actuator is secured to the housing 10 at a topportion 14, and the main valve 12 is secured to the guide member 4. Areturn coil spring 13 disposed surrounding the cylinder 8 is providedbetween the main valve 12 and the bottom of the frame 11. A bypass valve15 is slidably mounted on a shaft 14A secured to the cylinder 8 andresiliently held on the shaft 14A by a spiral spring 14a. The flange 16has a hole 19a (FIG. 4) for communicating the space (A' of FIG. 12) inthe thermostat housing with the passage (B' of FIG. 12) in thethermostat cap. The diameter of the hole 19a is determined to a value sothat the pressure of the coolant in the outer side passage of the mainvalve 12 (second passage 30 of FIG. 12) becomes equal to the pressure tothe coolant in the inner side passage, namely in the housing 10.

As a result, the spring constant of the return spring 13 can be reducedto a half of that of the conventional spring.

FIG. 2 shows the main valve in the close state. When the temperature ofthe coolant rises in excess of a predetermined value of the thermostat,the wax 7 in the heat conductive cylinder 8 expands. This forces theseal spool 5 against the rod 3. Since the rod 3 is secured to thehousing 10, the cylinder 8 is downwardly moved against the force of thespring 13, thereby opening the main valve 12, and closing the bypassport 32 with the bypass valve 15 (FIG. 3).

When the coolant temperature decreases, the wax contracts. Thus, thecoil spring 13 causes the main valve 12 to move to the closed position.

The operating range of the thermostat with respect to the temperature ofthe wax 7 is divided into a large lift up range of the main valve and asmall lift up range. In the large lift up range, the wax is in the solidstate where the volume of the wax largely changes. In the small lift uprange, the wax is in the liquid state where the volume of the liquid waxchanges at a small rate.

In order to increase the thermal efficiency of the automotive engine, itis necessary to increase the valve lift up rate with respect to thecoolant temperature, thereby reducing an upper limit of the coolanttemperature.

In the thermostat of the present invention, the thickness of the sealspool is extremely thin (thickness of between 25% and 5% of the diameterof the rod 3), so that the pressure of the wax for the lift up of thevalve is reduced. Furthermore, a small hole (19a) is formed in theflange of the thermostat, so that pressures applied to the outer sideand inner side of the main valve become equal to each other. Therefore,the spring constant of the return spring is reduced. As a result, thelift up rate is increased in a low temperature range.

As set forth above, the main valve of the present invention opens at 72°C. When the lift reaches 12 mm, the coolant temperature is 85° C. asshown by the line X in FIG. 1.

The flow rate at the lift of 6 mm of X which is in the solid wax statecorresponds to the flow rate at the lift of 12 mm of Y which is includedin the liquid wax state. Thus, the thermostat of the present inventionuses only 50% of the own power at the lift of 12 mm. Therefore, even ifan automobile mounting the thermostat of the present invention is drivenat 150 Km/h, there remains the power of 50%.

Since the flow rate of the coolant at 81° C. is the double of that ofthe conventional thermostat, the coolant temperature quickly decreases.Therefore, the coolant temperature is kept 81° C.

FIG. 5 shows a test machine for measuring the lift of the main valvewith respect to the pressure applied to the seal spool. In the machine,oil pressure is used instead of wax pressure.

A thermo-actuator 36 is attached in the test machine, cutting the heatsensitive cylinder so as to observe a resilient seal spool 39. The spacebetween the seal spool 39 and a rod 40, is fitted with a lubricating oil41. The seal spool 39 can be observed through openings 37 and atransparent plastic pipe 38. The test machine has a slidable rod 34downwardly urged by a spring 42. The top of the rod 34 contacts with arod 43 of a dial indicator (not shown). The oil is supplied from aninlet 35.

Table 1 shows actually measured values for the relation between oilpressure and the lift of the main valve by the test machine of FIG. 5.

                  TABLE 1                                                         ______________________________________                                        PRESSURE-LIFT                                                                          THERMO-     THERMO-     THERMO-                                               ACTUATOR    ACTUATOR    ACTUATOR                                              (A)         (B)         (C)                                          PRESSURE LIFT        LIFT        LIFT                                         Kg/cm.sup.2                                                                            mm          mm          mm                                           ______________________________________                                        0        0           0           0                                            10       0           0           0                                            20       0           0           0                                            30       0           0           0                                            40       0           0           0                                            50       0           0.4         0.4                                          60       0           1.5         2.6                                          70       0           2.8         5.0                                          80       0.6         6.2         7.8                                          90       1.6         9.5         10.0                                         100      2.5         10.0                                                     110      5.5                                                                  120      8.0                                                                  130      9.5                                                                  140      10.0                                                                 ______________________________________                                         SPRING CONSTANT OF RETURN SPRING: 0.55 Kg/mm                             

In the table 1, a thermo-actuator (A) has rod 3 of 3.8 mm diameter andseal spool 5 of 1.7 mm thickness (45% of the diameter), athermo-actuator (B) has the rod of 4.5 mm diameter and the seal spool of1.25 mm (25%), and a thermo-actuator (C) has the rod of 4.5 mm diameter,and the seal spool of 0.225 mm (5%). The spring constant of the returnspring 13 is 0.55 Kg/mm.

If the thickness of the seal spool 5 is extremely thin as thethermo-actuator (C), the pressure of the lubricant oil 6 in the sealspool becomes equal to the pressure of the lubricant oil 41. Since theresilient seal spool is in a floating state held by inner and outerequal pressures, the frictional resistance between the seal spool andthe rod becomes zero. The rod 3 is relatively lifted up by the pressureof lubricant oil 41 applied to the lower end face of the rod.

Since the thermo-actuator (A) has a large thickness of 1.7 mm, the liftis 0.6 mm at the starting pressure of 80 Kg/cm². In order to lift therod 10 mm against the spring load 15.1 Kg, the pressure of 140 Kg/cm² isnecessary, which is out of the question.

Although the starting pressure for the rod 3 is 50 Kg/cm² for both thethermo-actuators (B) and (C) and the lift is the same 0.4 mm, the rod of(C) is lifted up 10 mm by the pressure of 90 Kg/cm² because of extremelysmall thickness 0.225 mm. But for (B), the large pressure of 100 Kg/cm²is necessary.

If the thickness of the seal spool 5 exceeds the thickness of (B), thestarting pressure becomes larger than 50 Kg/cm². Therefore, the upperlimit of the thickness is 25% of the diameter of the rod 3.

The thickness of the seal spool of the thermo-actuator (C) issufficient. If the thickness becomes smaller, it is difficult tomanufacture such a thin seal spool, and the manufacturing costincreases. Therefore, the lower limit of the thickness is 5% of thediameter of the rod 3.

Table 2 shows actually measured values.

                  TABLE 2                                                         ______________________________________                                        PRESSURE- LIFT                                                                              THERMO-                                                                       ACTUATOR                                                                      (D)                                                             PRESSURE      LIFT                                                            Kg/cm.sup.2   mm                                                              ______________________________________                                        10                                                                            20                                                                            30            0.3                                                             40            4.0                                                             50            9.5                                                             60            13.5                                                            ______________________________________                                         SPRING CONSTANT OF RETURN SPRING: 0.27 Kg/mm                             

A thermo-actuator (D) has the same rod diameter and seal spool thicknessas the thermo-actuator (C), but the spring constant is reduced from theconventional value of 0.55 kg/mm to 0.27 kg/mm which is a half of theconventional value.

The rod is lifted up 0.3 mm at the starting pressure 30 kg/cm², and 13.5mm at the pressure 60 kg/mm². By reducing the thickness of the resilientseal spool to an extremely small value and reducing the spring constantto a half of conventional value, liquefaction of the wax is promoted, sothat the amount of the liquefied wax rapidly increases, thereby rapidlylifting up the main valve by the preeminent synergistic effect of thesmall thickness of the seal spool and the small spring constant.

Since the thermostat of the present invention operates at 50 percent ofown full power, it operates calmly and quickly, so that the engineoperates at small vibration and the life of the engine is extended.

Table 3 shows results of endurance tests of four thermostats of thepresent invention, and Table 4 shows results of endurance test of fourconventional thermostats.

                  TABLE 3                                                         ______________________________________                                                            DIFFERENCE FROM                                                               INITIAL STAGE                                                           VALVE OPEN        VALVE                                              CYCLE    TEMP.      LIFT mm                                                                              OPEN                                          No.  340 SEC. ° C.                                                                              AT 98° C.                                                                     TEMP.  LIFT                                   ______________________________________                                        1       0     87.2       10.74                                                     10,000   86.8       10.78  -0.4   +0.04                                       20,000   86.3       10.77  -0.9   +0.03                                       30,000   85.8       10.76  -1.4   +0.02                                       40,000   86.9       10.77  -0.3   +0.03                                  2       0     86.5       11.01                                                     10,000   86.0       11.02  -0.5   +0.01                                       20,000   85.8       11.00  -0.7   -0.01                                       30,000   86.0       11.02  -0.5   +0.01                                       40,000   86.6       11.00  +0.1   -0.01                                  3       0     87.4       10.57                                                     10,000   87.2       10.63  -0.2   +0.06                                       20,000   87.2       10.70  -0.2   +0.13                                       30,000   86.6       10.66  -0.8   +0.09                                       40,000   86.5       10.64  -0.9   +0.07                                  4       0     86.6       11.09                                                     10,000   85.6       11.12  -1.0   +0.03                                       20,000   85.6       11.16  -1.0   +0.07                                       30,000   85.8       11.14  -0.8   +0.05                                       40,000   86.4       11.16  -0.2   +0.07                                  ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                            DIFFERENCE FROM                                                               INITIAL STAGE                                                           VALVE OPEN        VALVE                                              CYCLE    TEMP.      LIFT mm                                                                              OPEN                                          No.  340 SEC. ° C.                                                                              AT 98° C.                                                                     TEMP.  LIFT                                   ______________________________________                                        1       0     80.6       8.05                                                      10,000   81.0       8.27   +0.4   +0.22                                       20,000   80.9       8.26   +0.3   +0.21                                       30,000   80.3       8.29   -0.3   +0.24                                       40,000   79.8       8.46   -0.8   +0.41                                  2       0     81.0       8.13                                                      10,000   82.0       8.13   +1.0   0                                           20,000   80.6       8.45   -0.4   +0.32                                       30,000   80.4       8.16   -0.6   +0.03                                       40,000   80.6       8.45   -0.4   +0.32                                  3       0     82.7       7.75                                                      10,000   82.5       7.85   -0.2   +0.10                                       20,000   82.1       7.78   -0.6   +0.03                                       30,000   82.7       7.45   0      -0.30                                       40,000   81.5       7.80   -1.2   +0.05                                  4       0     76.8       9.00                                                      10,000   78.3       8.93   +1.5   -0.07                                       20,000   78.5       8.60   +1.7   -0.40                                       30,000   81.5       7.95   +4.7   -1.05                                       40,000   82.6       8.07   +5.8   -0.93                                  ______________________________________                                    

Each of the test was performed 40,000 times by alternating a first testand a second. In the first test, a coolant of below 40° C. flows 120sec., and in the second test, coolant of over 98° C. flows 220 sec.

The most important factor for the durability of the thermostat is thevariation value of the lift. The variation of the present invention ismuch smaller than the conventional variation over one figure. Thedifference between the lift at the initial stage and that of the finalstage is almost zero.

By applying the following improvement to the thermostat of the presentinvention, it is possible to further increase the effect of thethermostat.

If the thickness of the resilient seal spool is reduced to an extremelysmall value, the inner capacity of the heat sensitive cylinderincreases, so that the wax pressure decreases. Consequently, it ispossible to reduce the thickness of the cylinder, which causes the innercapacity to be further increased. Therefore, the wax pressure can bereduced in the inverse proportion to the square of the diameter of therod by increasing the diameter. Thus, it is possible to further reducethe upper limit temperature 81° C. of the coolant temperature.

Furthermore, if a thermostat having wax X line (FIG. 1) which is meltedat 69° C. is used, the upper limit temperature decreases to 78° C.

In accordance with the present invention, the cooling fan 33 is providedto be started at a coolant temperature of 81° C. or less. To this end, acooling fan switch is provided in the cooling system of the engine.

FIG. 9a is a sectional view of a cooling fan switch, and FIG. 9b shows aside view showing the cooling fan switch of FIG. 9a at full-scale.

The cooling fan switch 44 is attached to a thermo-actuator 45 of a smallsize. The thermo-actuator 45 comprises a rod 46, a guide member 47slidably mounted on the rod 46. A seal spool 48 is slidably engaged withthe rod 46, and secured to the guide member 47. A lubricant oil 49 fillsin the space between the seal spool 48 and the rod 46. These three partsare assembled and inserted in a heat sensitive cylinder 51 filled withwax pellets 50. An end of the cylinder 51 is securely and hermeticallyengaged with the guide member 47 and inserted in a body 52 and securedthereto, thereby forming the thermo-actuator 45.

A stopper ring 53 is engaged in an annular groove formed in the rod 46at an upper portion thereof. A push rod 55 is engaged with the rod 46 ata lower end portion thereof, and a return spring 54 is provided betweena flange of the push rod 55 and an inside wall of the body 52, therebyabutting the underside of the flange against the stopper ring 53.

The push rod 55 is slidably engaged in a center hole of the body 52, andan upper portion of the push rod is projected from the body 52.

The cooling fan switch 44 comprises an insulation body 63 provided witha plus terminal 62 securely mounted in a case 57. A U-shaped conductiveplate 60 made of phosphor bronze, electrically connected to the plusterminal 62 and elastically held by a chin of a stopper 61. A fixedcontact 59 is fixed to the conductive plate 60. A movable contact 58 isfixed to a snap disk 64 made of phosphor bronze so as to confront thefixed contact 59. The snap disk 64 is curved so as to snap act. Aperipheral portion of the snap disk 64 is secured to the case 57 by astop ring 65 and a spring 66. An upper portion of the case 57 is pressedagainst the insulation body 63, thereby forming the snap action switch44.

When the temperature of the coolant rises, the wax 50 in the heatsensitive cylinder 51 expands. This forces the seal spool 48 against therod 46, so that the rod is upwardly moved. Thus, the push rod 55 israised against the spring 54, and hence the top 56 of the push rod 55pushes the movable contact 58 to contact with the fixed contact 59,thereby closing the snap switch.

When the coolant temperature decreases, the wax 50 in the heat sensitivecylinder 51 contracts. Thus, the coil spring 54 causes the push rod 55to downwardly move. Thus, the movable contact 58 is detached from thefixed contact 59 to open the switch.

The cooling fan switch 44 is attached to a wall of a coolant passage ata proper position by a screw thread 52a, so that the heat sensitivecylinder 51 is immersed in the coolant. Thus, the movable contact 58 isgrounded. The plus terminal 62 is connected to a plus terminal of thecooling fan motor 76 (FIG. 12). Therefore, when the coolant temperaturereaches 75.5° C., the cooling fan is started.

FIG. 10 is a diagram showing the change of the coolant temperature withrespect to the elapsed time. When the coolant temperature A (at A' ofFIG. 12) reaches 75.5° C., the cooling fan switch is closed so that thecooling fan is started. Therefore the coolant temperature reduces andrises, and hence, the temperature cyclically changes at a constantamplitude between 75.5° C. and a lower temperature as shown in thediagram. The temperature does not exceed 75.5° C. The temperature B isheld at about 75.5° C.

At the time when the cooling fan switch 44 is closed, the cooling fancontinues to rotate by the inertia thereof. Therefore, the torque forstarting the motor is very small, which results in reduction ofvibration and noises.

FIG. 11a is a sectional view of an IC cooling fan switch 67 with athermo sensor 72, and FIG. 11b is a side view of the switch atfull-scale. The IC cooling fan switch 67 comprises a body 71, case 69and plus terminal 70 secured to the insulation body 71a. In the body 71,an IC thermo sensor 72 is provided. A plus terminal 73 of the sensor 72is connected to the plus terminal 70, and a minus terminal 74 isconnected to the case 69. Insulation resin such as epoxy resin is pouredinto the inside of the switch 67 from an opening 75 to solidify theinside parts. The cooling fan switch is also closed at 75.5° C. forexample.

FIG. 12 is a cooling system according to the present invention. The sameparts as FIG. 6 are identified as the same reference numerals as thoseof FIG. 12.

A thermostat cap 26a is made into a structure so as to mount the ICcooling fan switch 67. The switch 67 is securely mounted on the cap 26aby screw thread 68, so that the sensing portion of the switch is locatedin the cap 26a. The plus terminal 70 is connected to a relay 75. Acontact of a relay switch 77 is connected to a drive motor 76 of thecooling fan 33. Thus, when the coolant temperature reaches 75.5° C., therelay switch 77 is closed. Hence the cooling fan 33 is started, and thecoolant temperature is held at 75.5° C.

The parts enclosed by a dot-dash line of FIG. 12, such as the water pump28, fan motor 76, cooling fan 33, thermostat 1 and radiator 22 areassembled into one-set.

The starting temperature of the cooling fan switch is set by such amanner that the starting temperature is lowered by 1° C. step by step,confirming the operation of the cooling system and the engine, and isdetermined to a temperature in a preferable operating condition.

In accordance with the present invention, by the synergistic effectdependent on the very thin thickness of the seal spool, the small springconstant of the return spring, the thermostat linked to the fan switch,the upper limit of the coolant of the present invention is reduced to81° C. or lower from 123° C. of the conventional thermostat, so that thefuel consumption of the engine is reduced, the life of the engine isextended, emission of No_(x) and CO₂ is decreased, thereby contributingto the prevention of the global warming.

While the invention has been described in conjunction with preferredspecific embodiment thereof, it will be understood that this descriptionis intended to illustrate and not limit the scope of the invention,which is defined by the following claims.

What is claimed is:
 1. A cooling system for an automotive enginecomprising:a radiator for cooling a coolant of the automotive engine; acooling fan driven by a motor for cooling the coolant in the radiator; afirst coolant passage provided between an outlet of a water jacket ofthe engine and an inlet of the radiator; a second coolant passageprovided between an outlet of the radiator and an inlet of the waterjacket; a bypass passage communicating the first coolant passage and thesecond coolant passage with each other; a housing provided at anintersection of the second coolant passage and the bypass passage; a rodsecured to the housing; a guide member slidably mounted on the rod; aresilient seal spool enclosing the rod and hermetically engaged with theguide member at a base portion thereof; a lubricant oil sealed in aspace between the rod and the seal spool; a heat sensitive cylinderenclosing the guide member and the seal spool;wax pellets sealed in theheat sensitive cylinder; a main valve secured to the guide member so asto close the second coolant passage; a bypass valve secured to the heatsensitive cylinder so as to close the bypass passage; a flange having avalve seat and secured to the housing; a return spring urging the mainvalve to the valve seat; and coolant pressure being applied to insideand outside surfaces of the flange when the main valve closes the secondcoolant passage; wherein; the thickness of the seal spool at a portionengaging with the rod is 25% to 5% of the diameter of the rod; at leastone hole is formed in the flange; a cooling fan switch is provided fordetecting the temperature of the coolant, and provided to be closed at apredetermined temperature; the thickness of the seal spool and thespring constant of the return spring are selected so that the operatingof the main valve is within a temperature range in which the wax pelletschange from a solid state to a liquid state; and the predeterminedtemperature for closing the cooling fan switch is set to a value so thatan upper limit of the coolant temperature is limited within a range inwhich the wax pellets change from a solid state to a liquid state.